Such processes are known from WO-A-9941332, U.S. Pat. No. 6,080,301, EP-A-0668342, U.S. Pat. No. 6,179,994 or WO-A-02070629. These processes all comprise some kind of hydroisomerisation of the Fischer-Tropsch synthesis product followed by a dewaxing step of the higher boiling fraction obtained in said hydroisomerisation.
WO-A-02070629, for example, describes a process wherein the C5 plus fraction of a Fischer-Tropsch synthesis product is first subjected to a hydrocracking/hydroisomerisation step in the presence of a catalyst consisting of platinum on an amorphous silica-alumina carrier. The effluent of this conversion step is separated into middle distillate products and a base oil precursor fraction, also referred to as a waxy raffinate product, and a higher boiling fraction. The base oil precursor fraction is catalytically dewaxed in the presence of a platinum-ZSM-S based catalyst and the heavy fraction is recycled to the hydrocracking/hydroisomerisating step.
It would be useful to improve the quality and the yield of the intermediate waxy raffinate product.